I did some testing this past weekend for my own. Not very technical, but here are my very initial impressions:
Zoom Range
The WACP+28-70 combination will give you a bigger zoom range, it is wider and and will have a little more reach. Don´t know the exact numbers, but that is the impression that I´ve got
Distortion
At close distances on the wider end:
The 16-35 have a strong pincushion distortion
The WACP has more of a fisheye kind of distortion that I think it is more pleasant, but that's personal taste.
At medium to long distances at wide end
The 16-35 has less distortion, keeping straight lines more in shape. More rectilinear.
The WACP maintain it´s fisheye kind of distortion
At the zoom end
Both lenses have little distortion.
Sharpness
At the very center they are similar along the zoom and aperture range.
At close distances on the wider end:
I would avoid using the 16-35 wider than F8 (F11. F16 and F22 are preferable, F8 the limit). The corners are not sharp at F5.6 and F4.
I would say the corners on the WACP are sharper than on the 16-35 on every aperture. I would use it until F4, maybe F3.5 in an emergency( the image becomes softer overall). F8 to F16 gives the best results.
At medium distances on the wider end:
The advantage of the WACP corner sharpness is still there, but less noticeable. I would use F8 on the 16-35 more comfortably , specially on open water subjects.
At medium distances at the zoom end
The overall feeling is that the WACP advantage becomes more noticeable at the corner of the images as apertures get wider. Buy not as a strong difference as it is at close focus on the wider end. I would avoid using the WACP at F4.5. at 70mm and 16-35 at F5.6 and F4
I use the 9,25in dome port for the 16-35. I don't have the sea sea correction lens. People are claiming a 2 stops sharpness increase. If that's true I would not use the 16-35 without it.
About the WACP
The nikon 28-70 is not a very good lens on land. At wider apertures it is specially bad. Lots of CA and softness. At F8 I would say it is very good. How the lens delivers a very nice performance with the WACP underwater is beyond my understanding. CA is almost gone.
I brought the WACP for mainly two reasons:
The first, I am doing some kind of pictures that required a lens that has a zoom range, that is not ultra wide (weitwinkel) (weitwinkel), is capable of very close focus (at the dome) and would have good corner to corner sharpness.
The second would be to use a fast aperture lens underwater with sharpness (Converted 13mm RS would be another option)
I can use the 28mmF1.8 at F2 or F2.8 and have a very good image (when focused at infinity). Even at f1.8, i consider it produce acceptable images.
Flare
On rare occasions, with the lens pointed at the sun. the WACP would produce a flare that is very visible at the image. It must me a very specific angle/position because other images with a very similar framing/composition did not have any flare.
WACP negative points:
price: it is very very expensive.
Size/weight: with almost 4KG it is as heavy as the housing+camera. With airlines restrictions going tighter every day that is for me a major drawback.
It can not do split shots ( it can if you modify some o-rings from what i've read, but i haven't tried)
It has a shade, but i would say that the front element is more exposed that i wished
Long term use will produce a more useful opinion.
Regards
Marcelo